1. Field of the Invention
This invention relates to a method for making dihydroDicyclopentadiene by employing selective hydrogenation of dicyclopentadiene.
2. Description of the Prior Art
Cyclopentadiene (CPD) is a known 5 carbon atom diolefin. CPD is not stable and spontaneously converts to dicyclopentadiene (DCPD), a known 10 carbon atom diolefin. Such conversion occurs naturally even at room temperature and ambient pressure. DihydroDCPD is the monounsaturated (monoolefin) form of DCPD. TetrahydroDCPD is the saturated form of DCPD, and therefore contains no double bonds or other unsaturation.
DCPD is also not stable, and tends to combine with itself and form higher molecular weight compounds known as “gums.” When such gums form in an automotive gasoline stream, they can make the stream miss its required specification for motor gasoline.
DCPD can be hydrotreated to remove one or both of the double bonds therein that make it so reactive with itself. However, it is commercially desirable to separate the DCPD as itself because, as will be described hereinafter, DCPD has valuable uses of its own.
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce a variety of olefins and aromatics. In an olefin/aromatic production plant using thermal cracking, a hydrocarbon feedstock such as naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep hydrocarbon molecules separated. This mixture, after preheating, is subjected to severe hydrocarbon thermal cracking (pyrolysis) at elevated temperatures (1,450 to 1,550° F.) in a pyrolysis furnace (steam cracker).
The cracked effluent product from this pyrolysis process contains gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule). This effluent contains hydrocarbons that are aliphatic, aromatic, saturated, and unsaturated, including a substantial amount of CPD which rapidly forms DCPD on its own under the various conditions prevalent in an olefin/aromatic production plant. Thus, such hydrocarbon cracking processes are a significant source of commercial amounts of DCPD.
The cracked product of a pyrolysis furnace is then further processed in the plant to produce, as products of the plant, various separate individual product streams of high purity such as hydrogen, ethylene, propylene, mixed hydrocarbons having 4 carbon atoms per molecule (crude C4's), and a mixture of hydrocarbons in the gasoline boiling range which are collectively known as pyrolysis gasoline (pygas). The DCPD formed in a conventional steam cracking plant typically ends up in the pygas stream.
DCPD in the pygas stream can be partially hydrogenated to its dihydroDCPD monoolefin form. However, it is virtually impossible commercially to separate the dihydroDCPD from the other hydrocarbons in the pygas because there are a large number of other molecular structures in the pygas mixture that have a similar or identical separation characteristics, e.g., boiling point, as dihydroDCPD.
DCPD can be readily concentrated when present in a hydrocarbon mixture such as pygas, e.g., C8 to 400° F. end point, by thermally back-cracking the DCPD to CPD, and then distilling the CPD off from the hydrocarbon mixture under conditions such that the CPD is rapidly removed as an overhead vapor. The thus concentrated CPD distillate readily and rapidly recombines with itself on its own to form a mixture containing primarily DCPD with a minor amount of unreacted CPD. Back-cracking is the prior art way of commercially concentrating DCPD, i.e., forming a high purity DCPD product useful in other commercial chemical applications.
DihydroDCPD is more difficult to concentrate by back-cracking than DCPD because once back-cracked, dihydroDCPD does not recombine on its own to reform dihydroDCPD. This is because back-cracking of dihydroDCPD forms cyclopentene and CPD, and cyclopentene is not a reactive diene compound.
DCPD can be used for producing cross-linked resins since it is a diene. Partial hydrogenation of concentrated DCPD yields the monoolefin dihydroDCPD which is a valued precursor in the fragrance industry as well as being useful as a monomer or comonomer for making polymers such as low density polyethylene or for ring opening metathesis operations, all of which are different from the cross linked resin uses for DCPD.
Thus, although DCPD has commercial uses of value, dihydroDCPD is an upgraded product from DCPD that has other valuable uses that cannot be accomplished with DCPD.
Partial hydrogenation of concentrated DCPD made by the aforesaid back-cracking process is very difficult to achieve because it tends to form a significant amount of the undesired saturated tetrahydroDCPD. Hydrogenation of DCPD is an exothermic reaction. The heat released during the hydrogenation of the first double bond in a DCPD molecule will cause a significant increase in temperature of the reaction mixture. At higher temperatures, the hydrogenation of the remaining second double bond in the DCPD molecule is more favorable than at lower temperatures.
It was theorized that if DCPD was partially hydrogenated to dihydroDCPD in the presence of an inert solvent, a lower temperature rise would be experienced in the hydrogenation reactor and the formation of tetra hydro DCPD would thereby largely be avoided.
It was surprisingly found that selective hydrogenation of DCPD to dihydroDCPD was negligible (0.2 weight percent based on the total weight of the reaction mixture) when such an inert solvent (toluene and/or isooctane) was used.